This disclosure relates to seismic exploration for oil and gas and, in particular but not by way of limitation, relates to seismic data processing using adaptive operators.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey may involve deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations, creating pressure changes and vibrations along the way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), while others are sensitive to particle motion (e.g., geophones); industrial surveys may deploy one type of sensor or both types. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
Some surveys are known as “marine” surveys because they are conducted in marine environments. However, “marine” surveys may not only be conducted in saltwater environments, but also in fresh and brackish waters. In one type of marine survey, called a “towed-array” survey, an array of seismic sensor-containing streamers and sources is towed behind a survey vessel. Other surveys are known as “land” surveys because they are conducted on land environments. Land surveys may use dynamite or seismic vibrators as sources. Arrays of seismic sensor-containing cables are laid on the ground to receive seismic signals. The seismic signals may be converted, digitized, stored or transmitted by sensors to data storage and/or processing facilities nearby, e.g. a recording truck. Land surveys may also use wireless receivers to avoid the limitations of cables. Seismic surveys may be conducted in areas between land and sea, which is referred to as the “transition zone”. Other surveys, incorporating both hydrophones and geophones, may be conducted on the seabed.
One of the goals of the seismic survey is to build up an image of a survey area for purposes of identifying subterranean geological formations. Subsequent analysis of the representation may reveal probable locations of hydrocarbon deposits in subterranean geological formations. However, before a desired image can be built, the acquired seismic data need to be processed, e.g. cleaned and re-conditioned. The desired signals are the ones that travel from a source, are reflected by a subsurface structure once and are received by a receiver. They are referred to as direct reflection signals. The direct reflection signals are used to build up an image. All other undesired signals or noises need to be removed from the acquired seismic data. Some of the undesired signals that are reflected by subsurface structures multiple times before reaching a receiver are referred to as “multiples”. Others that are reflected by air-water interface (ocean surface) at least once are referred to as “ghost” signals. Signals originating from sources other than the controlled seismic sources of the survey are noises. There are many different methods to process seismic data to obtain the desired seismic data.
Ghost signals from the sea surface cause constructive and destructive interferences in marine towed-streamer seismic data. A ghost signal that is reflected by the air-water interface first then by a subsurface geological layer may be referred to as a “source-side ghost.” A ghost signal that is reflected by a subsurface geological layer first then by the air-water interface may be referred to as a “receiver-side ghost.” Both the source-side and receiver-side ghosts need to be removed during marine seismic data processing. The process or method to remove ghost effects may be referred to as “de-ghost”. There are many ways to remove ghost signals, or “de-ghost”. For example, the ghost effects can be modelled and mitigated to some degree if the wavefield is well-sampled spatially and the receiver depths are known. Unfortunately if these conditions are not met, which is often the case, receiver-side deghosting, among other things, may be problematic.